1. Field of the Invention
The invention relates to thin film materials and devices made therefrom, and more particularly to methods of fabricating thin film with improved tear resistance.
2. Description of the Related Art
Thin film shape memory alloys (“SMA”) such as TiNi, also know as Nitinol, are used in many medical applications such as stents and stent covers. For some of these applications bulk material is not suitable due to its stiffness. It is difficult to roll TiNi film to a thickness less than about 30 microns, and the rolling process induces work hardening that makes the material hard and inflexible. Small diameter intravascular devices are made of thin films a few microns in thickness. These devices have the advantage of being extremely flexible, enabling their insertion by catheter into tortuous blood vessels. Foils of 30 microns or less in thickness are most practically made by sputter deposition.
It is generally known that TiNi SMAs lack the property of toughness. In particular, Nitinol is notch sensitive, meaning that a small crack (even microscopic in size) on the surface (and especially at the edge of a sheet) will propagate under stress, and this process is not “self healing” as it is in some other materials. Thin film is particularly vulnerable to crack propagation because it can be easily bent out of plane so that a shear stress becomes a tearing stress.
Improved tear resistance of TiNi thin films will increase their usefulness in many applications, and especially in applications that require long cycle life and in which a failure can be catastrophic. In some medical applications such as heart valve leaflets a tear could be fatal to a patient.
Most tears in thin film materials originate at an edge. A tear in a thin sheet begins with a small crack. Cracks propagate because the crack produces a concentration of stress at the tip of the crack. A well known method of stopping cracks in ductile materials (such as plastics) is to create a hole at the end of the crack. Drilling a hole through a sheet of plastic distributes the tear force over a longer path and thereby eliminates the local stress concentration
The need has therefore been recognized for fabrication methods that produce thin film materials having improved tear resistance over available prior art thin film materials. Heretofore there has not been provided a suitable and attractive solution to the problems of tearing in prior art thin film materials.